1. Field of the Invention
The present invention relates to a sliding member usable under sliding conditions, for example, in vacuum, high temperature, or ultraclean environment, where none of liquids, viscous materials, and solid lubricants can be used, and a process for producing the same.
2. Description of the Related Art
For example, when sliding members and rolling members, such as sliding bearings, seal mechanisms, bolts, nuts, pistons, crankshaft fulcrums, pins, cams, links, jack shafts, gears, shaft seal parts, and rolling element bearings, are used at high temperatures or under vacuum, two planes in pair cause relative movement while they are in contact with each other. This creates friction or microscopic collision between the surface of one of the members and the surface of the other member. When the pressure of contact between two planes (plane pressure) is high and the relative movement speed is high, local heat generation or scratch of the counter material by projections, damage by abrasion and the like occur. In extreme cases, seizing or adhesion occurs, often leading to suspension of operation.
In order to prevent these unfavorable phenomena, when sliding members are used under severe sliding conditions, a suitable level of lubricity should be required to act on a portion between two planes facing each other.
Here, in the case of sliding members to be used under ordinary environment, a liquid or a viscous lubricating oil (lubricant) may be provided between two planes in pair to acquire satisfactory lubricity. Thus, the two planes in pair are placed in a fluid film lubrication state or a boundary lubrication state. This can avoid adhesion between the two planes.
On the other hand, when a lubricating oil is used as lubrication between the sliding members, which should be unavoidably used under vacuum, high temperature, or ultraclean environment, the presence of the lubricating oil itself becomes an obstacle to maintain a desired environment, or otherwise the lubricating oil easily undergoes a change of properties which results in the disappearance of the desired lubricity. For this reason, lubricating oils cannot be used under vacuum, high temperature, or ultraclean environment. This makes it necessary to ensure the lubricity by materials other than liquid or viscous lubricating materials.
Various solid lubricants have hitherto been used as alternative to liquid or viscous lubricants. Representative examples thereof include {circle around (1)} graphite, {circle around (2)} compounds such as molybdenum disulfide, phthalocyanine, lead oxide, and boron nitride, and {circle around (3)} plastics. A suitable material is selected among these materials by taking into consideration the environment (for example, temperature, degree of vacuum, corrosiveness, chemical activity, pressure of the plane of contact, and sliding speed) and used. Since, these alternative materials have both advantageous and disadvantageous properties respectively, the use of them under severe conditions in such required environments is limited naturally.
For example, graphite can be used in the air without causing any problem at a temperature of about 400xc2x0 C., which is a higher temperature than where conventional lubricating oils can be used. Further, graphite is suitable for use where electrical conductivity is required. However, graphite is almost useless under vacuum environments. On the other hand, molybdenum disulfide can be used at a temperature of 1100xc2x0 or above under vacuum environment, and can be used in the air at a temperature of 300xc2x0 C. or below without undergoing oxidation. Molybdenum disulfide, however, has a low durability against repeated use for a long period of time. Phthalocyanine can withstand a temperature up to 600xc2x0 C., and lead oxide can withstand a temperature up to 500xc2x0 C. That is, these substances have features of high resistance to high temperatures. Plastics have a low coefficient of friction even in bulk. In particular, fluororesins have a very low coefficient of friction of 0.04, but on the other hand, due to the nature of the resins, the maximum temperature which they can withstand is 250xc2x0 C. (starting temperature of decomposition), which is very inferior compared to inorganic materials and metals.
Thus, there are a wide variety of coating-type lubricants for use in sliding members, ranging from liquids to solids, each of which have both advantages and disadvantages.
Accordingly, for sliding members used under severe conditions, instead of using lubricants, various attempts have been made to ensure the lubricity by thinly coating a soft metal on the sliding surface of hard metal.
FIG. 4 shows a general construction of this type of conventional sliding member, wherein one sliding surface 10a in a pair of sliding members 10, 12, is thinly coated with a soft metal layer 14 formed of a soft material such as lead, indium, gold, or silver. The sliding members 10, 12 are formed of a hard metal moved each other while they are in contact with the soft metal layer 14. This concept is based on such an idea that lubricating action is caused by the soft metal, in which low shearing stress of the soft metal is utilized.
That is, in this case, a true contact area A between the sliding members 10, 12 is determined by the hardness of a harder metal member (sliding member 12). When the contact surfaces once come into true contact with each other and followed by relative movement (friction) to cause shear separation of the two surfaces from each other, a breakage occurs in the thin soft metal layer 14. In particular, when any lubricating oil is absent between the two surfaces, a piece of soft metal 16 separated by the breakage is fast moved around suitably between the two contacting surfaces. Even though a part of the soft metal layer 14 has been breached, it has been found that a piece of soft metal 16 is transferred from other part onto this breached portion, thereby the breached portion is self-repaired. In this case, when lubricating oil is used therewith, it becomes difficult to transfer the fluid soft metal 16 to the breached portion. Therefore, self-repairing is hardly expected, and on the contrary, this sometimes causes inconvenience.
Here, silver as a soft metal is said to be the best material for use in high load slide bearings, because silver is highly compatible with the hard metal member, and at the same time, has good thermal conductivity and toughness.
Electroplating is generally used for coating a soft metal on a sliding member formed of a hard metal. For example, when a sliding member is plated with silver on a commercial scale, a commonly used method is such that plating is carried out using a plating solution composed mainly of silver potassium cyanide at a current density of not less than 20 A/dm2 by taking advantage of high stability of the cyan complex.
The electroplating of the sliding member on its sliding surface, however, suffers from the following drawbacks: (1) a special power supply unit should be generally provided; and (2) when the objects have complicated shapes or members which are very small, the current distribution is likely to be uneven, resulting in a variation in plating thickness or plating quality from portion to portion.
Accordingly, the electroplating can be satisfactorily brought its advantage only for mass-produced objects having a simple shape. By contrast, the electroplating cannot be satisfactorily used for members having a complicate shape and deformed members having a labyrinth contour. Also, the electroplating cannot be satisfactorily used for objects, which are required to execute the plating on customer""s site (in particular, for example, in the case of assembling, installation, and repair), but not within the production plant. Thus, the development of a method, other than electroplating, which can surely and easily form a soft metal layer on a sliding member in its sliding surface, has been strongly desired. Further, harmful effects of highly poisonous cyanogenic compounds contained in conventional plating solutions on the health of workers have also been strongly pointed out.
Incidentally, it has also been proposed to coat on rolling bearing parts or the like with silver by ion plating. Ion plating, however, requires the use of a much larger apparatus than electroplating apparatuses, and on-site works are likely to be impossible.
The present invention has been made in view of the above drawbacks. It is therefore an object of the present invention to provide a sliding member and a process for producing the same, wherein a high-quality soft metal layer is evenly coated on a sliding surface by a process alternative to electroplating or ion plating.
In order to attain the above object, according to an aspect of the present invention, there is provided a sliding member comprising: a sliding member having a sliding surface thereon; and a metal layer covering said sliding surface.
The metal layer has been formed by ultra-fine metal particles, which have been melted and bonded to one another. The ultra-fine metal particles are preferably ultra-fine silver particles and the average particle diameter of the ultra-fine silver particles is 1 to 20 nm, and in particular 1 to 10 nm.
According to another aspect of the present invention, there is provided a process for producing a sliding member having a metal layer on its sliding surface, which comprises: providing an ultra-fine particle solution of ultra-fine metal particles dispersed in a predetermined solvent; bringing the ultra-fine particle solution into contact with said sliding surface of said sliding member; drying the ultra-fine particle solution deposited on said sliding surface to form dried coating; and heat treating said dried coating to melt and bond the ultra-fine metal particles to one another.
By virtue of this constitution, the sliding surface can be evenly covered with a thin metal (for example, silver) layer of orderly arranged pure ultra-fine metal particles produced by decomposing, through firing. The whole organic matter has been contained in a solvent with ultra-fine metal particles being homogeneously mixed and dispersed therein. The whole metal coating process can be carried out in the air at room temperature to about 200-300xc2x0 C. Thus, a sliding surface of the sliding member can be covered with a metal layer at much lower cost in a simpler manner with higher reliability.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.